feat: Test running overhaul, switch to Prettier & reformat everything (#1407)

* chore: Switch to Node 20 + Vitest

* chore: migrate to vitest mock functions

* chore: code style (switch to prettier)

* test: re-enable long-running test

Seems the switch to Node 20 and Vitest has vastly improved the code's and / or the test's runtime!

see #1193

* chore: code style

* chore: fix failing tests

* Updated Documentation in README.md

* Update contribution guidelines to state usage of Prettier

* fix: set prettier printWidth back to 80

* chore: apply updated code style automatically

* fix: set prettier line endings to lf again

* chore: apply updated code style automatically

---------

Co-authored-by: github-actions <${GITHUB_ACTOR}@users.noreply.github.com>
Co-authored-by: Lars Müller <34514239+appgurueu@users.noreply.github.com>

.github/workflows/Ci.yml

@@ -15,20 +15,15 @@ jobs:
 
       - uses: actions/setup-node@v3
         with:
-          node-version: 16
+          node-version: 20
           cache: npm
 
       - name: 📦 Install dependencies
         run: npm ci
 
       - name: 🧪 Run all tests
-        if: ${{ github.event_name == 'push' }}
         run: npm run test
 
-      - name: 🧪 Run tests for changed files only
-        if: ${{ github.event_name == 'pull_request' }}
-        run: npm run test-changed
-
       - name: 💄 Code style
         run: npm run style